The present invention relates to an all-differential operational amplifier, to be implemented as a so-called "internal" amplifier in monolithic integrated circuits manufactured with the CMOS technique, typically for integrators.
Among operational amplifiers built-in in monolithic integrated circuits, only very few are intended to drive a load which is external to the circuit. For these amplifiers (so-called "output buffers"), the load is unpredictable both in nature and magnitude (as an example, resistive to less than 1 kiloohm or capacitive to hundreds of picofarads), and furthermore they must be able to withstand a wide common-mode signal on the inputs.
Most operational amplifiers employed in MOS integrated circuits, instead, drive internal loads, which are defined exactly during the design phase, often of a merely capacitive type (of a few picofarads): these are so-called "internal" operational amplifiers, which furthermore are not necessarily requested to withstand common-mode voltages on the inputs. Typically, an amplifier of this kind is used as an integrator. From these operational amplifiers, the following main properties are expected:
(a) a high open-loop gain; PA1 (b) predisposition to drive merely capacitive loads; PA1 (c) short settling time; PA1 (d) wide excursion of the output signal; PA1 (e) low dissipation of power; PA1 (f) low equivalent input noise; PA1 (g) high rejection of disturbances superimposed on the power supply; PA1 (h) small occupied area.
Recently the use has spread of amplifiers with a differential output, in which the output signal is not composed of the difference between a single voltage and a fixed reference (as in single-ended amplifiers), but of the difference between the two outputs, which turn out to be symmetrical with respect to said fixed reference. This reference is set to a value which is equidistant from the values of the power supplies. In these amplifiers, an important problem is therefore that of designing a feedback loop which forces the common-mode of the output voltage to a voltage which is close to the reference, so that the excursion of the output can be symmetrical.
The use of all-differential amplifiers has spread mostly because in this way a much better rejection of the power supply disturbances is obtained, and also because an excursion of the output signal is available which is double with respect to the one which can be obtained with single-ended amplifiers.
Some embodiments of all-differential operational amplifiers are described in the article "MOS Operational Amplifier Design--A Tutorial Overview", by P. R. Gray and R. G. Mayer, in IEEE Journal of Solid - State Circuits, vol. SC-17, No. 6, December 1982.
In the NMOS technology, these amplifiers are manufactured in two stages, to obtain the high gain required. Examples of these two-stage amplifiers are also found in the CMOS technology. These two-stage amplifiers have the disadvantage of requiring a compensating capacitor which is proportional to the load capacity; this compensation, besides occupying a not insignificant area, can be critical for the stability of the amplifier, and furthermore varies according to the specific situation in which the amplifier is employed, since the load will be generally different. The compensating capacity also extends the settling time as the load capacity increases.
It is known to overcome these disadvantages in an integrated differential amplifier, by employing a differential input stage followed by a "folded cascode" stage, which is the actual stage where the gain is developed, i.e. where the high impedance node is located. In this way the compensating capacitor is eliminated, thus eliminating the stability problems, but the settling time turns out to be extended in any case by the load capacity itself.